1. Field
This disclosure relates generally to semiconductor devices, and more specifically, to sigma-delta modulators with trimmed quantizer reference voltage.
2. Related Art
Analog-to-digital converters (ADC's) are employed in a variety of demanding applications including computer systems, wireless telephones, and power metering systems. Such applications demand cost-effective ADC's that can efficiently convert an analog input signal to digital output signal over a wide range of frequencies and signal magnitudes with minimal noise and distortion.
An ADC typically converts an analog signal to a digital signal by sampling the analog signal at predetermined sampling intervals and generating a sequence of binary numbers via a quantizer in response thereto. The sequence of binary numbers is a digital signal representation of the sampled analog signal.
The length of a binary number assigned to a given sampled value of the analog signal corresponds to the number of quantizer bits and is limited. Consequently, a digital sample will not always precisely represent the corresponding analog sample. The difference between a digital sample and the corresponding analog sample represents quantization error.
The sampling frequency of the ADC is the inverse of the sampling interval. The resolution of the ADC is directly related to the number of binary bits assigned to each sampled value and inversely related to quantization error. The minimum difference between successive values that may be represented by the quantizer is the quantization step size. Quantization error results in quantization noise in the output digital signal, which is also called granular noise.
One measure of the performance of the sigma-delta modulator is the signal to noise and distortion ratio, which is a ratio of the power of a portion of the output signal that excludes quantization error, noise and distortion to the power of the sum of quantization error, noise and distortion and is expressed in decibels. The higher the number the number of decibels, the better the performance of the sigma-delta modulator. The quantization error is introduced by rounding or truncating the difference between the analog signal and the digital signal. One way to improve performance is to use quantizers with higher numbers of detection levels. The higher levels of detection in quantizers have increased complexity and require a greater number of components. Yet it is desirable to reduce the number of components required to achieve the desired performance.